Quadrature modulation is a technique for transmitting a communication signal. In quadrature modulation, a transmitter simultaneously transmits data on an in-phase (I) channel and a quadrature-phase (Q) channel. Each of the I and Q channels carry a separate data stream and are phased-shifted by 90 degrees relative to one another on a carrier frequency. In an I/Q receiver, the I and Q channel signals are received on the carrier frequency, down-converted and demodulated to recover the data from the separate I and Q channels.
The I/Q receiver includes separate analog processing paths for each of the I and Q channels. Each of the I and Q channel paths includes processing components that process the received analog signal and convert it to digital form, such as, for example, mixers, analog to digital converters (ADCs), amplifiers, and filters. These separate components down-convert and process the channel signal data for each of the paths. The use of the separate I and Q analog processing paths in the I/Q receiver results in what is known as I/Q imbalance, which is the phase and amplitude mismatch between the I and Q channel signals. The major source of I/Q imbalance in I/Q receivers is mismatch between the separate components in each of the I and Q channel processing paths.
There are two types of I/Q imbalance: frequency-independent mismatch and frequency-dependent mismatch. Frequency-independent mismatch is phase mismatch caused the local oscillator (LO) in the mixer of each I/Q analog path. These mismatches are similar across the frequency spectrum and, further, can be successfully estimated for error correction. Frequency-dependent mismatch is the phase and amplitude mismatch caused by the baseband components in each of the I and Q channel analog paths. The frequency-dependent mismatch complicates the mismatch estimation technique because the mismatches are not uniform across the frequency spectrum.
Techniques have been developed to estimate the mismatch parameters for the analog paths of an I/Q receiver. These techniques include real-time estimation, in which the receiver estimates mismatch parameters and compensates for the mismatch during real-time use of the receiver. The techniques also include off-line estimation, in which the receiver is not in use when estimation of mismatch is performed and correction parameters are input to the receiver. Off-line estimation methods may use tone injection techniques to inject test tones into the receiver at the inputs to the I and Q channels to calibrate mismatch between the channels using a method of tone calibration. The techniques have been shown to be most effective in the low-noise environment and, further, when the jitter of the test tone signals is lower than a certain level. In high-noise level environments or when test tone signal jitter increases above a certain level, the effectiveness of these known techniques decreases. It would provide an advantage, therefore, to have a method for estimating I/Q channel mismatch that had improved effectiveness in noisy environments and/or at high jitter levels in the test tone signals.